Proportions of demersal fish exposed to sublethal hypoxia revealed by otolith chemistry

Fishes exposed to hypoxia may experience sublethal effects that impair growth, reproductive fitness, and condition, with potential consequences for population sustainability. Characterizing sublethal effects of hypoxia exposure requires the ability to differentiate between exposed, and non-exposed individuals. Moreover, because sublethal effects may manifest well after exposure to hypoxia has ceased, long-term markers that facilitate retrospective identification of exposure are necessary. We used sagittal otoliths from 337 Atlantic croaker Micropogonias undulatus sampled at multiple stations in the northern Gulf of Mexico to characterize exposure to hypoxia during the first year of life. Otolith elemental profiles of Ba:Ca and Mn:Ca revealed 4 primary clusters of fish associated with residence in normoxic, hypoxic, and estuarine habitats. Croaker exposed to hypoxia in the first year of life constituted 34% of all individuals sampled in fall of 2014, and this pattern was consistent across the sampling region. Young-of-year croaker exposed to hypoxia were smaller in length and mass but had similar mean relative condition factors to croaker of the same year class experiencing normoxic conditions. These results indicate that in some years, one-third of the young-of-year croaker in the northern Gulf of Mexico experience hypoxic conditions and survive. The potential for sublethal effects of hypoxia on growth may be important for future efforts to forecast population sustainability under chronic seasonal hypoxia in the region.